I am not expert on Mandarin, or linguistics in general, but can share with you what I've learned;. Eng Wai is correct to say there are various types of spoken dialects all classed under the linguistic nomenclature called "Mandarin". Just as Cantonese is one of many Yue dialects, Yue is the family of dialects of which Cantonese is one dialect.

Mandarin dialects vary across the width of China. Most southern Chinese languages and their dialects are found in the south east. If you imagine China is a square which has been formed of four smaller squares, the north west, north east, and south west are the major areas where Mandarin dialects are found. Mandarin in Beijing is different from Mandarin in Sichuan. That is to say, that Sichuan has it's own local variety of dialect, but not the same as Beijing's speech patterns. From James Campbell's glossika site

Have a look at the list of tones for the dialects of Mandarin that he's gathered, and you'll see that some have still got their Ru tones which is different from Beijing which has lost hers, and the national language, putonghua is based upon the speech patterns of Beijing.

With regards to when Mandarin arises, scholars believe that the precursor of modern beijinghua may have began around the Yuan or Mongol dynasty.

The reason for this is that during the preceeding Song Dynasty, a number of character dictionaries were printed which maintained the use of the Ru tone, but only a hundred and fifty or so years later came the Mongol conquest, and shortly into the Yuan dynasty, the famous ZhongYuan YinYun or Rimes of the Central Plains was published expressly for the use of riming in Yuan period operas. In this work, all the Ru tone characters became distributed in Yin Ping, Yang Ping, Shang and Qu tone rimes, meaning the -p, -t, -k ending characters had been lost. However, it still maintained -m and -n ending rimes. This is known in the literature as Early Mandarin.

So I would guess that because language changes over a few generations, the Song-Yuan boundary is probably the time era in which Mandarin arises.

Mind you, you must look at other factors. The population of the north was severly depopulated during the middle to end of the Song dynasty, with people and their families fleeing southwards. That is why you see more dialects and languages in Southern China retaining the endings of earlier times.

Then Dyl, what do you suggest for the mandarin word of"mandarin (language group)"? The glossika site URL has suggested Guan Hua, instead of Bei fang hua.

Just a point of interest, in Malaysia, everyone use Hua2 yu3 for pu tong hua. So we have "duo1 jiang3 hua2 yu, shao3 shuo1 fang1 yan2" campaign. Hua yu, Chinese language, has been equated to pu tong hua, and regional languages, fang yan, has been equated to dialects.

Another point of interest, i actually don't really like the word Pu tong hua. I feel that it is a very awkward mandarin vocabulary, a straight translation of common language. In mandarin, seldom there a term is consisting of 2 words adjective and 1 word noun. We say mei3 nu3, not piao4 liang4 nu3 etc. And I cant recall any pu tong + noun example. Normally 2 words adjective is coined with 2 words nouns. Zhoung hua ming zu, not zhong hua zu. Hua zu, not zhong hua zu. Zhong guo ren, ma lai xi ya ren, xiang gang ren etc are exceptions because zhong guo, ma la xi ya are countries/region names.

I think Michael A. Fuller's (An Introduction to Literary Chinese) use of "Guanhua" is good enough, and distinguishes

Bei Guanhua, XiBei Guanhua, XiNan GuanHua and Nan Guanhua

as the various groupings of Mandarin dialects. Whether or not the populous would use these terms is another thing. Just as in English the linguistic term for Chinese languages is 'Sinitic', Guanhua can be taken as the Chinese linguistics term.

'Putonghua' is a term for a specific form of Mandarin. It is the 'national language' and because it is used throughout China in schools, it is the 'common language' of the people, or those who have recieved an education in Chinese.

I can't help you about your own perceptions of 'Mandarin' and its language. It's one of those things, you can take it or leave it.